Bill Gates & Paul Allen Supply an Operating System for the IBM PC

In December 1980 IBM hired Paul Allen and Bill Gates of Microsoft, then in Bellevue, Washington, to create an operating system (OS) for the new IBM personal computer under development.

Because Microsoft had no OS at the time, they purchased a non-exclusive license to sell a CP/M clone called QDOS ("Quick and Dirty Operating System") from Tim Patterson of Seattle Computer Products for $25,000.